Editing Magnification (section)

===Telescope===
The angular magnification of an [[optical telescope]] is given by

<math display="block">M_\mathrm{A}= {f_\mathrm{o} \over f_\mathrm{e}}</math>

in which <math display="inline">f_\mathrm{o}</math> is the [[focal length]] of the [[objective (optics)|objective]] [[lens (optics)|lens]] in a [[refracting telescope|refractor]] or of the [[primary mirror]] in a [[reflecting telescope|reflector]], and <math display="inline">f_\mathrm{e}</math> is the focal length of the [[eyepiece]].

====Measurement of telescope magnification====
Measuring the actual angular magnification of a telescope is difficult, but it is possible to use the reciprocal relationship between the linear magnification and the angular magnification, since the linear magnification is constant for all objects.

The telescope is focused correctly for viewing objects at the distance for which the angular magnification is to be determined and then the object glass is used as an object the image of which is known as the [[exit pupil]]. The diameter of this may be measured using an instrument known as a Ramsden [[dynameter]] which consists of a Ramsden eyepiece with micrometer hairs in the back focal plane. This is mounted in front of the telescope eyepiece and used to evaluate the diameter of the exit pupil. This will be much smaller than the object glass diameter, which gives the linear magnification (actually a reduction), the angular magnification can be determined from

<math display="block">M_\mathrm{A} = {1 \over M} = {D_{\mathrm{Objective}} \over {D_\mathrm{Ramsden}}}\,.</math>